Lactic acid bacteria are classified as heterotrophic chemoorganotrophs, meaning that they require preformed organic carbon as a source of both carbon and energy.
Lactic acid bacteria also lack cytochrome or electron transport proteins, and therefore cannot derive energy via respiratory activity. Thus, substrate-level phosphorylation reactions that occur during glycolysis (see below) are the primary means by which ATP is obtained. There are, however, other means by which these organisms can conserve energy and save ATP that would ordinarily be used to perform necessary functions, such as nutrient transport (see below).
Although there are some important differences between how various genera and species use and metabolize specific carbohydrates, lactic acid bacteria generally lack metabolic diversity and instead rely on two principal pathways for catab-olism of carbohydrates. In the homofermentative pathway, hexoses are metabolized via enzymes of the Embden-Meyerhoff pathway (Fig. 1), yielding 2 mol of pyruvate and 2 mol of ATP per mole of hexose. Pyruvate is subsequently reduced to lactate by lactate dehydrogenase, so that more than 90% of the starting material (i.e., glucose) is converted to lactic acid. The NADH formed via the glyceraldehyde-3-phosphate dehydrogenase reaction is also reoxidized (forming NAD+) by lactate dehydrogenase, thus maintaining the NADH/NAD+ balance. Among lactic acid bacteria used as dairy starter cultures, most are homofermen-tative, including Lactococcus lactis, Streptococcus thermophilus, Lactobacillus helveticus, and Lb. delbrueckii subsp. bulgaricus.
In heterofermentative metabolism, hexoses are catabolized by the phospho-ketolase pathway (Fig. 2), which results in approximate equimolar production of lactate, acetate, ethanol, and CO2. Only 1 mol of ATP is made per hexose. In actuality, however, product yields for both homo- and heterofermentative metabolism can vary, depending on the source and amount of available substrate, growth temperature, atmospheric conditions, and other factors. Under certain conditions, for example, some homofermentative organisms can divert pyruvate away from lactate and toward other so-called ''heterolactic'' endproducts (see below). Importantly, the pathway used by a particular strain or culture may have a profound effect on flavor, texture, and overall quality of fermented dairy products. Although several species of Lactobacillus are heterofermentative, Leuconostoc spp. are the only heterofermentative lactic acid bacteria used as starter cultures in dairy products.
As described earlier, lactic acid bacteria generally rely on either the Embden-Meyerhoff or phosphoketolase pathway for metabolism of sugars. In fact, these catabolic pathways are only a part of the overall metabolic process used by these bacteria. The first, and perhaps most important, step in carbohydrate metabolism involves transport of the sugar substrate across the cytoplasmic membrane and its subsequent accumulation in the cytoplasm. This process of transport and accu-
mulation is important for several reasons. First, active transport of sugars requires energy, and much of the energy gained by cells as a result of catabolism must then be used to transport more substrate. Second, the transport system used by a particular strain dictates, in part, the catabolic pathway used by that organism. The transport machinery also plays a regulatory role and can influence expression glucose hexokinase
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